Field of the Invention
This invention generally relates to linear displacement measuring devices with capacitive sensing and more particularly to such devices known as “dial indicators.”
Description of Related Art
Broadly, linear displacement measuring devices, such as dial indicators, measure position by determining a linear displacement of a spring-loaded spindle rod. Early implementations of such dial indicators included precision gearing to translate the linear spindle motion into rotation of a needle over a dial. More recently, efforts have been directed to reducing the mechanical complexity of such dial indicators by incorporating electronics, particularly digital electronics, to increase measurement accuracy and operational reliability.
One prior art approach incorporates capacitive sensors to generate position dependent measurements. In such a dial indicator a first stationary support carries an array of signal transmitting electrodes and forms a “slide” or “scale.” A second support undergoes relative motion with respect to the first support and carries receiving electrodes. The receiving electrodes and second support form a “scale” or “slide” or “reader” that undergoes motion relative to the stationary support. In the following discussion “scale” designates the first or stationary support and transmitting electrodes; “reader” designates the second or movable support and receiving electrodes.
The reader electrodes and scale electrodes are closely spaced in an overlying relationship forming capacitors in which the capacitance between the scale electrodes, or “bars,” varies as the reader moves. Processing electronics convert the received signals into a position measurement and display the position of the spring-loaded spindle relative to a reference position.
Prior art dial indicators that utilize capacitive sensing provide relative position measurements where the indicator is positioned at a convenient reference point and a “zero” button is pressed. All measurements then are relative to this position until the unit is powered down or re-zeroed. A dial indicator using relative position measurements does not “know” actual position of the spindle, just its relative motion. With absolute position measurement, the indicator “knows” the exact position of its spindle at all times. A relative zero can still be set, and it is not lost if the unit is powered down.
At times the velocity of the reader and spindle relative to the scale can increase and overload the operating capabilities of processing electronics thereby introducing ambiguities and potentially affecting measurement accuracy. Some prior art dial indicators overcame this problem by providing a dial indicator with “absolute” position sensing.
Other prior art dial indicators utilize independent precise and simultaneous measurements of multiple input signals. However, such systems can be sensitive to manufacturing tolerance variations. One solution includes one set of electrodes to make fine measurement and a second set of electrodes to make coarse measurements. Such arrays are difficult to implement and necessitate construction of electrode arrays to tolerances that are difficult to achieve. As a result, such dial indicators with the ability to provide absolute position are expensive to manufacture.
Electronic dial indicators are subject to both the effects of mechanical and electronic tolerances. Factory calibration is particularly important when implementing absolute positioning. Consequently, it is advantageous to provide a means for combining calibration information and actual measurement data being processed during a measurement operation. For ease of use, dial indicators can be battery operated. Prior dial indicators with absolute and incremental position measurement capabilities need to be continuously energized during measurements so that a zero reference is not lost and thus have power requirements that can shorten battery life and thus require either periodic battery replacement or battery recharging. These characteristics have reduced the demand for such measurement devices because battery-operated prior art devices have a short battery life.
What is needed is a dial indicator with capacitive sensors that provides accurate readings for relative and absolute positions, that is easily calibrated and that extends battery life.